This invention relates to an inverter for exciting a synchronous machine from a source of direct current potential, and more specifically to an inverter capable of providing forced commutation for initial machine excitation.
In many industrial applications where a polyphase synchronous machine is to be employed to provide rotational energy, it is often desirable to excite the machine from a DC source through an inverter. Inverters used in such machine applications are typically comprised of a plurality of pairs of serially-connected controlled unidirectional conductor means, such as thyristors, corresponding in number to the number of machine phases, coupled in parallel across a DC source. Each pair of thyristors is coupled at the pair junction to a corresponding phase of the polyphase machine, respectively. During operation, each of the inverter thyristors is sequentially gated into conductor such that during any one interval, a diagonally opposite pair of thyristors is conductive, thus permitting current conduction in a pair of corresponding machine phases for machine excitation. Induced back electromotive force (EMF) of the polyphase machine serves to commutate, that is to say extinguish current conduction through, each thyristor as a function of machine load variation. By appropriate feedback means, the inverter thyristor gating frequency can be adjusted as a function of load variation to allow adjustment of the frequency of the alternating voltage exciting the machine to achieve machine-inverter synchronization.
To allow load commutation to occur, it is necessary to initially accelerate the synchronous machine to a speed at which load commutation can be achieved. This is usually accomplished by providing additional circuitry to force inverter commutation for an initial time interval.
In the past, inverter circuits of the type described above often included a pair of auxiliary controlled unidirectional conduction means, such as thyristors, serially coupled across the DC source and connected at their pair junction through a commutating capacitor to the machine neutral terminal for causing forced commutation. During intervals of forced commutation, each of the auxiliary thyristors together with predetermined main thyristors were appropriately sequentially gated into conduction to conduct current through a pair of corresponding machine phases, thus providing alternating current sequentially through pairs of machine phases for machine start-up. To reduce current surge caused by the simultaneous conduction of one of the auxiliary thyristors and a diagonally opposite pair of main thyristors, a resistor was used to limit inverter link current drawn from the DC source.
The major disadvantage of such previous circuits is that inverter current is limited by a resistor during forced commutation intervals, thus dissipating energy in the resistor. In addition, limiting the inverter current in the manner of prior art forced commutating circuits requires the value of the commutating capacitor to be relatively large in order to store sufficient charge to commutate each of the inverter thyristors at predetermined intervals.
The present invention concerns an inverter circuit for providing forced commutation which is not subject to the disadvantages of the prior art circuits as enumerated above.